Chemical Ionization (CI) Mass Spectrometry

Electron impact is the conventional method for ionizing sample molecules in mass spectrometry. However, for some molecules, the high-energy electron beam causes excessive fragmentation of the primary molecular ion.

The absence of a molecular ion peak in such mass spectra makes identifying the sample difficult.

An alternative chemical ionization method is the gas-phase protonation reaction, where first, the relatively stable conjugate acid of the sample molecule is created. This species then fragments into low-molecular-weight components.

For instance, consider the chemical ionization of di-sec-butyl ether. The ether is mixed with methane gas. A high-energy electron beam then ionizes methane preferentially.

The methane radical cation formed collides with another methane molecule, generating a methanium ion and a methane radical.

The methanium ion is a superacid and source of a gas-phase proton, which protonates the ether. Compared to the molecular ion formed by electron-impact ionization, the protonated ether is stable, giving the M+1 peak in the mass spectrum.